The field of the present invention is the manufacture of a polymer in a gas-phase and fluidized-bed reactor.
The main problems in gas-phase and fluidized-bed polymerization processes are often related to the entrainment outside the fluidized bed of fine particles of the polymer in the course of formation. This phenomenon, when it is poorly controlled, is often the source of numerous problems, such as the fouling, or even the blocking of the pipes and various devices. Numerous efforts have thus been directed at the use of means which seek to eliminate or to reduce these problems related to the entrainment of particles outside the bed.
A solution which has been known for a long time consists in widening the transverse cross section in the upper part of the reactor in order to reduce the velocity of the gas stream above the fluidized bed and thus to promote the sedimentation of the entrained solid particles. The most widely known means is to use a cylindrical reactor surmounted by a disengagement chamber with a greater transverse cross section. However, this solution is not very economical and does not make it possible to completely eliminate the entrainment of solid particles.
Generally, all the means which promote the sedimentation of particles entrained above the fluidized bed are insufficient and do not completely eliminate the problem of the entrainment of particles and in particular of fine particles.
To at least partially solve the problems relating to the entrainment of the particles, it is necessary to use other techniques in parallel with the above-mentioned solutions, such as separation techniques using, for example, dust separators, such as cyclones.
In the majority of cases, the separators used are combined with one or more injection devices which make it possible, by virtue of suction by the Venturi effect generated using a carrier gas, to introduce the separated particles into the polymerization reactor. French Patent FR 2,618,786 discloses a gas-phase and fluidized-bed polymerization process for which the fine particles entrained by the gas stream above the fluidized bed are separated and are brought into contact with a carrier gas in a mixing device and are reintroduced as a mixture with this gas into the reactor above the fluidized bed. The said mixing device corresponds to the description of the above device, which is generally known under the name of ejector or of ejector/compressor. The use of such a device in a polymerization process presents a risk related to its fouling by the polymer particles. Furthermore, this type of device is often limited in terms of throughput of solid particles and may generally only be viable if the entrainment is already reduced by a widening of the effective transverse cross section above the fluidized bed. Furthermore, the experience has shown that an acceleration of the fine particles to a higher velocity before reintroduction info the reactor is not viable because it is a major contributor to static build-up in the reactor bed.
French Patent Application FR 2,137,441 also discloses the use of an ejector as described above but the solid particles are reintroduced directly into the fluidized layer. This improvement with respect to the preceding patent does not, however, eliminate the problems mentioned above which are related, inter alia, to the fouling and to the blocking of the said ejectors.
European Patent EP 0,381,364 discloses a fluidized-bed reactor comprising a vertical cylinder for which a dust separator is connected to the reactor via a pipe for recycling the fine particles, this pipe emerging in the upper half of the cylinder. The claims refer to a figure clearly exhibiting the use of an ejector, that is to say of a device for mixing between the separated particles and a carrier gas, as described above.
The separators used can be combined with diplegs inside the fluidized bed. This type of device makes it possible to inject the separated particles into the fluidized bed. This technique is commonly used in steam cracking processes of FCC type for the circulation of the catalyst particles. This is characterized by a dense-phase transport in a tubular pipe of the solid particles as far as the inside of the fluidized bed, the said tubular pipe generally being inclined at its end. The main advantage of this device is related to its simplicity of use. Once again, this device is difficult to apply to a polymerization process because of the risks of fouling, or even blocking, due to the dense phase in the transport of active solid particles.
There exists a need for a person skilled in the art to discover a technique for recycling the solid particles entrained above a fluidized bed which makes it possible to solve the abovementioned problems.